Metal nanostructured networks and transparent conductive material

ABSTRACT

Metal nanowires, such as silver nanowires coated on a substrate were sintered together to form fused metal nanowire networks that have greatly improved conductivity while maintaining good transparency and low haze. The method of forming such a fused metal nanowire networks are disclosed that involves exposure of metal nanowires to various fusing agents on a short timescale. The resulting sintered network can have a core-shell structure in which metal halide forms the shell. Additionally, effective methods are described for forming patterned structure with areas of sintered metal nanowire network with high conductivity and areas of un-sintered metal nanowires with low conductivity. The corresponding patterned films are also described. When formed into a film, materials comprising the metal nanowire network demonstrate low sheet resistance while maintaining desirably high levels of optical transparency with low haze, making them suitable for transparent electrode, touch sensors, and other electronic/optical device formation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part application of U.S. patentapplication Ser. No. 13/530,822 filed on Jun. 22, 2012 to Virkar et al.,entitled “Metal Nanowire Networks and Transparent Conductive Material”and claims priority to U.S. provisional patent application Ser. No.61/684,409 filed on Aug. 17, 2012 to Virkar et al., entitled “MetalNanowire Films with Good Conductivity and Transmission with Low Haze”,both incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to fused networks of metal nanowires that aresuitable for the formation of electrically conductive and transparentfilms, such as for use as transparent electrodes. The inventions arefurther related to chemical methods for fusing the nanowires to formnetworks as well as to devices incorporating the fused metal nanowirenetworks.

BACKGROUND

Functional films can provide important functions in a range of contexts.For example, electrically conductive films can be important for thedissipation of static electricity when static can be undesirable ordangerous. Optical films can be used to provide various functions, suchas polarization, anti-reflection, phase shifting, brightness enhancementor other functions. High quality displays can comprise one or moreoptical coatings.

Transparent conductors can be used for several optoelectronicapplications including, for example, touch-screens, liquid crystaldisplays (LCD), flat panel display, organic light emitting diode (OLED),solar cells and smart windows. Historically, indium tin oxide (ITO) hasbeen the material of choice due to its relatively high transparency athigh conductivities. There are however several shortcomings with ITO.For example, ITO is a brittle ceramic which needs to be deposited usingsputtering, a fabrication process that involves high temperatures andvacuum and therefore is relatively slow and not cost effective.Additionally, ITO is known to crack easily on flexible substrates.

SUMMARY OF THE INVENTION

In a first aspect, the invention pertains to a precursor ink thatcomprises a solvent, at least about 0.01 weight percent metal nanowiresand from about 0.05 mM to about 50 mM halide anions. In someembodiments, the precursor ink can comprise from about 0.025 to about 2weight percent metal nanowires and from about 0.25 mM to about 10 mMhalide anions. In some embodiments, the metal nanowires in the precursorink are silver nanowires. The silver nanowires can have an averagediameter of no more than about 75 nm and a length of at least about 5microns. In some embodiments, the solvent of the precursor ink cancomprise alcohol and/or water. In some embodiments, the inventionpertains to a method for forming a transparent conductive film thatcomprises depositing a precursor ink described herein and drying thedeposited ink to form the film.

In a second aspect, the invention pertains to a material that comprisesa transparent conductive coating and a substrate on which the coating issupported. The coating comprises a sintered metal nanowire network thatcomprises sintered metal nanowires. The coating has a transparency tovisible light of at least about 90%, a sheet resistance of no more thanabout 500 Ohms/square, and a haze of no more than 0.5. In someembodiments, the coating of the material has a sheet resistance of nomore than about 200 Ohms/square. The substrate of the material cancomprise a polymer film. In some embodiments, the sintered metalnanowires of the material comprise sintered silver nanowires. Thesintered metal network comprises a metal halide shell layer. In someembodiments, the transparent conductive coating of the material can havea transparency to visible light of at least about 95%, a surface loadinglevel of the sintered metal network from about 0.1 micrograms/cm² toabout 5 mg/cm² and a haze of no more than about 0.4.

In a third aspect, the invention pertains to a material that comprises atransparent conductive coating and a substrate on which the coating issupported. The coating comprises a sintered metal nanowire network witha metal halide shell layer having an average thickness of at least about1 nanometer over a metallic core. In some embodiments, the sinteredmetal network of the material comprises sintered silver nanowires. Thecoating of the material has a transparency of at least about 90% ofvisible light and a sheet resistance of no more than about 200Ohms/square. In some embodiments, the sintered silver nanowires of thematerial have an average diameter of no more than about 75 nm and anaverage length of at least about 5 microns and the material has asurface loading level of the sintered silver nanowires from about 0.1micrograms/cm² to about 5 mg/cm².

In a fourth aspect, the invention pertains to a patterned structure thatcomprises a substrate and a coating, wherein the coating is patternedwith un-sintered metal nanowires in a selected portion of the coatingand a sintered metal nanowire network over another portion of thecoating. The coating portion with the sintered metal nanowire networkhas a sheet resistance of no more than about 500 Ohms/square and theportion with un-sintered metal nanowires has a sheet resistance at leastabout 5 times less than the sheet resistance of the sintered metalnanowire network. The coating of the patterned structure has atransparency to visible light of at least about 85% and the coating hasapproximately the same transparency across the entire coating. In someembodiment, the un-sintered metal nanowires in the coating of thepatterned structure comprise silver nanowires. The un-sintered metalnanowires have a sheet resistance at least about 1000 times the sheetresistance of the sintered metal network

In a fifth aspect, the invention pertains to a method for forming apatterned structure that comprises a substrate and a metal nanowirecoating. The method comprises selectively contacting a selected portionof the metal nanowire coating with a sintering agent to form a patternedcoating with the selected portion having a sheet resistance at leastabout 5 times greater than the sheet resistance of unselected portion ofthe metal nanowire coating. In some embodiments, the selectivecontacting of the method comprises directing a sintering vapor to theselected portion of the coating to form the patterned structure. In someembodiments, the method further comprises blocking the sintering vaporwith a mask over the unselected portion of the coating. In otherembodiments, the selective contacting of the method comprises directinga solution comprising a sintering agent at the selected portion of thecoating to form the patterned structure.

In a sixth aspect, the invention pertains to a touch sensor thatcomprises a first electrode structure and a second electrode structurespaced apart in a natural configuration. The first electrode structuregenerally comprises a first transparent conductive electrode comprisinga first sintered nanostructured metal network on a first substrate. Insome embodiments, the second electrode structure can comprise a secondtransparent conductive electrode comprising a second sinterednanostructured metal network on a second substrate. The first electrodestructure and the second electrode structure can be spaced apart by adielectric layer and the electrode structures can be connected to acircuit to measure changes in capacitance. In some embodiments, thetouch sensor can further comprise display components associated with thesubstrate. The substrates of the touch sensor can be transparent sheets.The touch sensor may comprise a circuit connected to the electrodestructures that measures changes in electrical resistance orcapacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing of metal grid based transparent electrodeformed through a traditional patterning approach.

FIG. 1B is a schematic drawing of a nanowire (NW) based transparentconductive material fabricated from low cost solution processablemethods disclosed herein.

FIG. 1C is a schematic drawing illustrating the process of threenanowires being fused together to form an elongated nanowire with twoangles around the fused points.

FIG. 1D is a schematic drawing illustrating nanowires being fusedtogether to form a fused NW based transparent conductive material withangles around the fused points and arrows indicating the formation of NWnetwork.

FIG. 1E is a schematic diagram showing the process of bonding nanowireswith sintering agent.

FIG. 1F is a schematic diagram of a capacitance based touch sensor.

FIG. 1G is a schematic diagram of a resistance based touch sensor.

FIG. 1H is a schematic diagram of a AgNW film with one sintered pattern.

FIG. 1I is a schematic diagram of a AgNW film with three sintered areas.

FIG. 2 is a plot of sheet resistance of the samples from the firstvendor tested before and after the HCl vapor treatment having atransparency at 550 nm greater than 75%.

FIG. 3 is a plot of sheet resistance of the samples from the secondvendor tested before and after the HCl vapor treatment showing dramaticimprovement in conductivity.

FIG. 4 is a plot of sheet resistance of the samples tested before andafter the HCl vapor treatment having a transparency at 550 nm greaterthan 85%.

FIG. 5 is a scanning electron micrograph (SEM) of silver nanowiresbefore any treatment.

FIG. 6 is a SEM micrograph of silver nanowires after heat treatment.

FIG. 7 shows six SEM micrographs of fused silver nanowires after HClvapor treatment.

FIG. 8 is a plot of sheet resistance of samples treated with 5 mM NaClin ethanol and AgF in ethanol solutions.

FIG. 9 is plot of conductivity of the film samples before and after thesintering from Example 4 plotted in logarithmic scale showing dramaticimprovement in conductivity after the treatment with the vapor sinteringagent.

FIGS. 10A and 10B are plots of conductivity of the film samples beforeand after the sintering respectively from Example 5 plotted inlogarithmic scale.

FIGS. 11A and 11B are plots of conductivity of the film samples beforeand after the sintering respectively from Example 6 plotted inlogarithmic scale.

FIGS. 12A and 12B are XPS (X-Ray Photoelectron Spectroscopy) of sample42 before and after the sintering respectively.

FIG. 13A is SEM of sample 42 before sintering.

FIGS. 13B-13D are SEM of sample 42 at different magnifications aftersintering.

FIG. 14 is a plot of the optical absorption of a sample on PET showingthe difference in surface plasmon resonance measurements of samplesbefore and after HCl vapor sintering.

FIG. 15 is a plot of conductivity of the treated and untreated areas offilm samples from Example 8 plotted in logarithmic scale.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A fused or sintered metal nanowire network can be formed chemically toachieve a structure with desirably low electrical resistance and hightransparency to visible light. The fused metal nanowire network can beformed as a coating for use as a transparent conductive layer. Silvernanowire can be a convenient material to form the network, but othermetal nanowires are also suitable for forming the network of fused metalnanowires. The chemical fusing or sintering can be performed using gasphase or solution phase ionic inorganic compositions with halogenanions. A solution with halogen anions can be delivered to a metalnanowire coating or combined with a nanowire dispersion for the directapplication as a coating with the metal nanowires co-deposited with thehalogen sintering agent. Since the electrically conductive network canbe formed at low temperature, the networks are suitable for use withmaterials, such as polymers, that cannot tolerate high temperatures. Ithas been found that some of the processing approaches can be adapted forefficient patterning with sintered regions and un-sintered regions alongthe surface to provide for selected electrical conduction pathways alongthe surface while providing good optical transparency across thesurface. Thus, the conductive networks are well suited to certaintransparent electrode applications and the low quantities of materialsand low temperature processing can provide for convenient commercialapplications. The sintered metal nanowire networks can form transparentelectrically conductive films with low haze, which is significant formany applications.

Metal nanowires can be formed from a range of metals. For example, theproduction of a range of metal nanowires is described, for example, inpublished U.S. patent application 2010/0078197 to Miyagishima et al.,entitled “Metal Nanowires, Method for Producing the Same, andTransparent Conductor,” incorporated herein by reference. There has beenparticular interest in silver nanowires due to the high electricalconductivity of silver. With respect to the specific production ofsilver nanowires, see for example, published U.S. patent application2009/0242231 to Miyagisima et al., entitled “Silver Nanowire, ProductionMethods Thereof, and Aqueous Dispersion,” and U.S. 2009/0311530 to Hiraiet al., entitled “Silver Nanowire, Production Method Thereof, andAqueous Dispersion,” and U.S. Pat. No. 7,922,787 to Wang et al.,“Methods for the Production of Silver Nanowires,” all three of which areincorporated herein by reference. Silver nanowires are commerciallyavailable, for example, from Seashell Technologies, LLC, CA, USA.

Silver is known to have a bulk melting point of about 960° C. However,nanoparticles of silver can melt at temperatures less than 150° C. Thismelting point depression observed for the nanoparticles are believed tobe based on the large surface area/volume ratios of the nanoparticles.In general, the larger the surface area/volume ratio, the greater theexpected mobility of the surface atoms, and the lower the melting point.Melting points of about 150° C. for silver nanoparticles however maystill be too high for a variety of substrates including plastics andelastomers. The time required for melting and cooling can also be inexcess of several minutes, which add time and process costs toproduction.

To produce flexible transparent conductive material that can be producedat reasonable cost and in large scale such as roll-to-roll coating orink jet printing method, numerous new materials have been developed asreplacements for indium tin oxide (ITO). A potential ITO replacement isa metal-grid shown in FIG. 1A. Metal grids, which can be formed usingpatterning approaches such as photolithography, can achieve very highperformances with low sheet resistances. However, the metal grid filmsare not solution-processable for example with roll-to-roll coating andtherefore are costly to fabricate and often involve fabrication methodswhich are difficult to scale. While the performance of metal grids mayexceed ITO, cost and processability are still hindering theirwide-spread adoption.

As shown schematically in FIG. 1B, the metal nanowires deposited into afilm from a dispersion can appear to be randomly arrayed rods thatintersect with each other randomly, although in practice some alignmentof rods can take place depending on the deposition process. While metalnanowires are inherently electrically conducting, the vast majority ofresistance in the silver nanowires based films is believed to due to thejunctions between nanowires. To improve the properties, it has beenproposed to embed the metal nanowires in a secondary electricallyconductive medium, see published U.S. patent application 2008/0259262 toJones et al., entitled “Composite Transparent Conductors and Methods ofForming the Same,” incorporated herein by reference. In principle, thejunction resistance of a AgNW network can be reduced by sintering orfusing the wires together using heat as disclosed in “Modeling themelting temperature of nanoparticles by an analytical approach.” by A.Safaei et. Al. in J. Phys. Chem. C, 2008, 112, 99-105 and in “Sizeeffect on thermodynamic properties of silver nanoparticles” by W. Luo etal. in J. Phys. Chem. C, 2008, 112, 2359-69. The heat can be appliedconventionally or by a light source. However, conventional heating maynot be practical for many applications since the NWs are not expected tomelt until 300-400° C., which is significantly greater than the thermalstability limits of most plastic substrates. Light sources can also beused, but may involve setup of additional and expensive equipment in alarge roll-to-roll fabrication. A room or low temperature process whichfuses or sinters the NWs is therefore highly desirable. Hu et al.disclosed similar results in ACS Nano, Vol 4, No. 5, 2010, 2955-2963entitled “Scalable coating properties of flexible, silver nanowireelectrodes.” Hu et al demonstrated that the junction resistance betweenthe silver nanowires can be in the giga-ohm range, but with processingto 110° C. with the optional addition of significant pressures for shorttimes improved electrical conductivity performance could be obtained.

Low temperature in combination with the application of pressure has beenused to achieve a significant decrease in electrical resistance whilereasonable levels of transparency were reported. See, De et al., “SilverNanowire Networks as Flexible, Transparent Conducting Films: ExtremelyHigh DC to Optical Conductivity Ratio,” ACS Nano Vol. 3(7), pp 1767-1774(June 2009). The De et al. article does not suggest that fusing of thesilver nanowires takes place, and the low temperature used in theprocess would seem to be too low to result in fusing. The process in theDe et al. article involved vacuum filtering and transfer using 100° C.and significant amounts of pressure for 2 hours. This process is notdesirable from a commercial processing perspective.

As described herein metal nanowires are sintered or bonded together atroom temperature, or more generally at temperatures less than about 80°C., to produce materials comprising bonded metal nanowire networks thathave greatly decreased sheet resistance relative to the unbonded metalnanowire structures. The sintering chemistry forms a core-shellstructure that has improved conductivity and transmission compared tothe unbound nanowires. The metal cores of adjacent nanowires aresintered together to form a bond and a shell of metal halide covers thewires along the network structure. Due to the metal halide coating, thecore-shell material can have altered optical properties. In particular,a lower reflectivity results from the metal halide shell that mayimprove optics for certain applications. Alternatively, the metal halideshell can be dissolved following sintering of the metal nanowires toremove any optical effects of the shell. The core shell structure can beunderstood based on the elucidation of the chemistry resulting in thesintering and can be confirmed through examination of micrographs, asdescribed further below. Referring to FIG. 1E, nanowires 1 and 3 withpolymer coatings 5 and 7 respectively are bonded together to form acore-shell structure 11 that has a bond junction point 13, a metalliccore 15 and a metal halide shell layer 17 over the metallic core. Themetal halide shell layer generally is believed to have an averagethickness of at least about 1 nanometer. Scanning electron micrographs(SEM) of exemplary bond junction point are described below in theexamples. Silver nanowires in particular has been found can be fusedtogether to improve the sheet resistance of the film formed from the10⁵-10⁸ or greater Ω/sq range to the 10 to 100 Ω/sq range with less than0.5% changes to the transparency. Nanowire network thicknesses can beused that provide overall good transparency of at least about 85% fornetworks with the reported low electrical resistance. The fusing can beachieved in less than a minute that impose little or no change or damageto the morphology of the metal nanowires. Thus, the process is wellsuited to efficient and relatively inexpensive commercial processing.

It was recently demonstrated by Magdassi and co-workers that thick filmsof silver nanoparticles (AgNPs) can be “sintered” at room temperatureusing various chemical agents for non-transparent patterned silver pasteapplication. A process for the chemical fusing of metal nanoparticles isdescribed in published PCT application WO 2010/109465 to Magdassi etal., entitled “Process for Sintering Nanoparticles at Low Temperatures,”incorporated herein by reference. The nanoparticle low temperaturesintering is further described in Grouchko et al., “Conductive Inks witha “Built-In” Mechanism That Enables Sintering at Room Temperature,” ACSNano Vol. 5 (4), pp. 3354-3359 (2011). The fusing of nanoparticles formsa sheet of metal, which can have a desired low electrical resistance,but the sheet of metal generally does not have desired amounts oftransparency.

A vapor based process for the formation of a conductive film from silvernanowires is described in Liu et al., “Silver Nanowire-BasedTransparent, Flexible and Conductive Thin Film,” Nanoscale ResearchLetters, Vol. 6(75), 8 pages (January 2011) (hereinafter “the Liuarticle”). The films formed as described in the Liu article hadreasonably low electrical resistance, but the transparency of the filmswas not satisfactory for many applications. The Liu article attributedtheir observations to the removal of surface oxidation from the silvernanowires. However, significant deterioration of the nanowire morphologyhas been observed in the micrographs shown in the Liu article. Liuarticle did not disclose conducting the reaction in solution phase withsolution based sintering agents or using fluoride based sinteringagents. Improved processing leads to significantly improved results forthe fused metal nanowire networks described herein. It is not clear ifsintering took place under the harsher conditions as noted in the Liuarticle. In particular, desired levels of fusing have been achieved withshort time processing of the nanowires with the halide anions withoutdegrading the level of optical transparency and with littledeterioration of the nanowire morphology.

Based on the results described herein, the mechanism of the lowtemperature sintering process has been examined. While not wanting to belimited by theory, it is believed that the halide ions form a surfacecoating that facilitates metal ion migration. At locations whereadjacent nanowires are close, the metal cores surprisingly fuse in asintering process that presumably is driven by a free energy reductionfrom the sintering that is accessible due to the cation migrationenabled by the formation of a metal-halide shell layer. The sinteredmetal forms electrical conduction pathways that results in dramaticdecreases in the electrical conductivity. The sintering of the metalnanowires is observed to not measurably change the transparency tovisible light. Thus, the chemical sintering of the metal nanowires intoa conductive network can be accomplished without measurably diminishingthe transparency so that the resulting films can be effectively used toform transparent conductive films.

For transparent electrode applications, higher-aspect ratio structureslike wires or tubes are advantageous since the rod like shape canpromote electrical conductivity primarily in-plane. The primarilyin-plane conductivity in these rod-like structures allows for “open”areas and thin films which are useful for high light transmission andgood 2D sheet conductivities. Nanowires (NWs) are particularly goodcandidates for transparent conductor applications. However due to theirmuch larger size of about 10s of nanometers in diameter and 10s-100s ofmicrons in length, the surface area/volume ratio of nanowires isconsiderably smaller than NPs. Silver NWs for example typically do notmelt until the temperatures of about 300-400° C. Silver nanowires are10⁴-10⁵ times larger in volume relative to nanoparticles and have muchsmaller ratio of surface area to volume and ratio of surface atoms tobulk atoms. The significant differences in physical size of nanowiresrelative to nanoparticles imply that the properties are likely to becorrespondingly different.

The improved fused/sintered metal nanowire networks described herein canachieve simultaneously desirably low sheet resistance values whileproviding good optical transmission. In some embodiments, the fusedmetal nanowire networks can have optical transmission at 550 nmwavelength light of at least 85% while having a sheet resistance of nomore than about 100 ohms/square. In additional or alternativeembodiments, the fused metal nanowire networks can have opticaltransmission at 550 nm of at least 90% and a sheet resistance of no morethan about 250 ohms/sq. Based on the ability to simultaneously achievegood optical transparency and low sheet resistance, the fused metalnanowire films can be used effectively as transparent electrodes for arange of applications. The loading of the nanowires to form the networkcan be selected to achieve desired properties.

To achieve the desirable properties of the fused metal nanowirenetworks, it was surprisingly discovered that short time exposure ofsilver nanowires to halide containing sintering agents coulddramatically improve the conductivity of the nanowire networks or films.In general, the metal nanowire networks can be exposed to the sinteringagent for times of no more than about 4 minutes to cause the desiredfusing/sintering, and in some embodiments significantly less time can beused as described further below. The dramatic reduction in sheetresistance may partially be attributed to the removal of the insulatingcapping polymer polyvinylpyrrolidone (PVP) that is used to stabilizecommercial silver nanowires, but is believed to be primarily related tothe sintering of the nanowires. SEM studies of the treated silvernanowires indicated clearly the formation of fusing points between thenanowires that are in close proximity of each other as well assignificantly reduced amount of detectable PVP polymer. In comparison,the SEM of the untreated silver nanowires clearly shows the presence ofPVP polymer and the gap between the ends of the closely situated silvernanowires. Referring to FIG. 1C, a schematic diagram illustrating theprocess of the ends of three adjacent nanowires being fused together isshown. The fused nanowires form an elongated nanowire with two anglesaround the fused points. Elongated nanowires can further form a networkof elongated nanowires as shown in FIG. 1D, with angles around the fusedends and arrows indicating the connection formation between theelongated nanowires to form the nanowire network.

To improve the performance of metal nanowire films generally, it ispossible to increase the length of the nanowires and/or tocorrespondingly decrease the diameter of the nanowires. As the length ofthe nanowires increase, longer conduction pathways are present withoutthe need for conduction across junctions between nanowires. As thediameter decreases, the overall film haze decreases, improving theoptical properties of the film. With the use of sintering agentsdescribed herein, a film can be formed with particular initial nanowireshaving significantly greater electrical conductivity based on aparticular loading with a corresponding optical transparency. Of course,with the availability of higher quality initial nanowires, such ashaving a longer average length and/or a smaller average diameter, asomewhat better un-sintered film may be formed in terms of electricalconductivity for a given loading or transparency, but the sinteringprocess correspondingly farther improves the electrical conductivitybeyond an improvement based on the properties of the metal nanowires.

The fused silver nanowires disclosed herein have considerabledifferences from the sintered silver nanoparticles disclosed by Magdassiand the treated silver nanowires disclosed by Liu. Specifically, afterthe sintering process, the AgNPs of Magdassi aggregated together. Theprofiles of the individual AgNPs that existed prior to the sinteringprocess have been destroyed considerably during the sintering process toform the aggregates. The word sintering indeed is a proper descriptionof the melting and coalescing, and or coarsening of the silvernanoparticles of Magdassi. With regard to the treatment of silvernanowires proposed by Liu, although Liu intended to improve conductivityof the silver nanowires by removal of AgO, the prolonged HCl treatmentdisclosed by Liu caused observable thinning and shortening of the silvernanowires that seems to have degraded the properties of the resultingmaterial.

In contrast to the processing approach described by Magdassi et al., thecurrent processing approach is directed to the production of networkswith a high level of optical transparency. The processing conditions aredesigned to achieve this objective, and the nanowire morphology isconducive to processing to obtain a desired degree of transparency. Inparticular, conductive films can have an optical transparency evaluatedfor convenience at 550 nm light wavelength of at least about 85%.

The processing of the metal nanowire networks described herein comprisesthe contact of a thin metal nanowire layer, i.e., a network, with achemical fusing agent comprising a halide anion. The fusing agent can bedelivered as a vapor or in solution. For example, acid halides aregaseous and can be delivered in a controlled amount from a gas reservoiror as vapor from a solution comprising the acid halide. Halide salts canbe dissolved in solution with a polar solvent with a moderateconcentration, and a quantity of the salt solution can be contacted withthe nanowire network to fuse adjacent nanowires. Suitable solvents forforming a solution with the chemical fusing agent include, for example,alcohols, ketones, water, or a combination thereof. It has beendiscovered that superior properties of the fused network results fromshort processing times for the nanowire network with the fusing agent.The short processing times can be successful to achieve very low levelsof sheet resistance while maintaining high optical transparency.

While the processing conditions are designed to produce good opticaltransparency, the metallic grid-like properties of the fused elongatedsilver nanowires dramatically increased conductivity with little changein transparency relative to the unfused networks. The drop in electricalresistance may be due to a drop in junction resistance between adjacentnanowires due to fusing of the adjacent nanowires. The treatmentdescribed herein may also have improved the connection between the otherconnecting points indicated by the arrows in FIG. 1D by removingoxidation layer of the nanowires, by removing the capping agent such asPVP of the nanowires, or by at least partially fusing these connectionpoint together. Although removal of the PVP from the surface of silvernanowires have been observed in the examples below, fusing of the silvernanowires at points of contact can also be observed. The finaltransparent conductive material can best be described as a fused networkof silver nanowires, as illustrated in FIG. 1D. The fused metal nanowirenetwork structure has advantages over conventional metal grids describedin FIG. 1A due to low cost fabrication methods and solutionprocessability.

In summary, a highly conductive and transparent material was formed atroom temperature by fusing the ends of silver nanowires to improve theconductivity without sacrifice the transparency. The resulting materialappears to be a silver metallic grid like structure that is highlyconductive. The examples below described using HCl as the fusing agentfrom the vapor phase, dilute solutions of HCl, NaCl, AgF, LiF, and NaFwere also used to create the materials of comparable properties at roomtemperature. It is understood the metal nanowires could be treatedmultiple times to achieve the desired degree of fusing, with the same ordifferent fusing agent during each treatment. Although silver nanowireswere used to perform the fusing experiments, it is understood that othermetal nanowires can be similarly fused together to form materials withimproved conductivity.

The transparent conductive films that are formed from the fused metalnanowires are suitable for various applications. For example, some solarcells are designed to have an electrode along the light receivingsurface, and a transparent conductive electrode may be desirable alongthis surface. Also, some display devices can be made with a transparentconductive electrode. In particular, touch inputs can be effectivelyformed with the transparent conductive films described herein, and theefficient patterning of the fused nanowire films can be used to formcorresponding patterned touch sensors.

Touch inputs or sensors generally operate based on change of capacitanceor a change of electrical resistance upon touching of the sensorsurface. A common feature of the touch sensors generally is the presenceof two transparent conductive electrode structures in a spaced apartconfiguration in a natural state, i.e., when not being touched orotherwise externally contacted. For sensors operating on capacitance, adielectric layer is generally between the two electrode structures.Referring to FIG. 1F, a capacitance based touch sensor 101 comprises adisplay component 103, an optional bottom substrate 105, a firsttransparent conductive electrode structure 107, a dielectric layer 109,such as a polymer or glass sheet, a second transparent conductiveelectrode structure 111, optional top substrate 113, and measurementcircuit 115 that measures capacitance changes associated with touchingof the sensor. Referring to FIG. 1G, a resistance based touch sensor 131comprises a display component 133, an optional lower substrate 135, afirst transparent conductive electrode structure 137, a secondtransparent conductive electrode structure 139, support structures 141,143 that support the spaced apart configuration of the electrodestructures in their natural configuration, upper substrate 145 andresistance measuring circuit 147.

Display components 103, 133 can be LED based displays, LCD displays orother desired display components. Substrates 105, 113, 135, 145 can betransparent polymer sheets or other transparent sheets. Supportstructures can be formed from a dielectric material, and the sensorstructures can comprise additional supports to provide a desired stabledevice. Measurement circuits 115 and 147 are known in the art, and somespecific sensor embodiments are referenced below in the context ofpatterning. Transparent conductive electrodes 107, 111, 137 and 139 canbe effectively formed using sintered metal networks, although in someembodiments the sintered metal networks form some electrode structureswhile other electrode structures in the device can comprise materialssuch as indium tin oxide, aluminum doped zinc oxide or the like. Fusedmetal networks can be effectively patterned as described herein, and itcan be desirable for incorporate patterned films in one or more of theelectrode structures to form the sensors such that the plurality ofelectrodes in a transparent conductive structure can be used to provideposition information related to the touching process. Patterning isdiscussed further below.

Electrically Conductive Film Structure and Properties

The conductive films described herein generally comprise a substrate anda fused metal nanowire network deposited on the substrate. An optionalpolymer coating can be placed over the metal nanowire network to protectand stabilize the fused nanowire network. The parameters of the metalnanowires can be adjusted to achieve desirable properties for the fusednetwork. For example, a higher loading of nanowires can result in alower electrical resistance, but transparency can decrease with a highernanowire loading. Through a balance of these parameters, desirablelevels of electrical conductivity and optical transparency can beachieved. The nanowires in the improved networks are fused, as isobserved in scanning electron micrographs. It is believed that thefusing of the nanowires results in the improved electrical conductivitywhile maintaining high levels of optical transparency. Having a networkwith fused nanowires should provide a stable electrically conductivestructure over a reasonable lifetime of a corresponding product.

In general, the nanowires can be formed from a range of metals, such assilver, gold, indium, tin, iron, cobalt, platinum, palladium, nickel,cobalt, titanium, copper and alloys thereof are desirable due to highelectrical conductivity. Silver in particular provides excellentelectrical conductivity, and commercial silver nanowires are available.To have good transparency, it is desirable for the nanowires to have asmall range of diameters. In particular, it is desirable for the metalnanowires to have an average diameter of no more than about 250 nm, infurther embodiments no more than about 150 nm, and in other embodimentsfrom about 10 nm to about 120 nm. With respect to average length,nanowires with a longer length are expected to provide better electricalconductivity within a network. In general, the metal nanowires can havean average length of at least a micron, in further embodiments, at least2.5 microns and in other embodiments from about 5 microns to about 100microns, although improved synthesis techniques developed in the futuremay make longer nanowires possible. An aspect ratio can be specified asthe ratio of the average length divided by the average diameter, and insome embodiments, the nanowires can have an aspect ratio of at leastabout 25, in further embodiments from about 50 to about 5000 and inadditional embodiments from about 100 to about 2000. A person ofordinary skill in the art will recognize that additional ranges ofnanowire dimensions within the explicit ranges above are contemplatedand are within the present disclosure.

Following sintering of the metal nanowires into a network, theindividual nanowires are no longer present, although the physicalproperties of the nanowires used to form the network are directlyreflected in the properties of the sintered network. While not wantingto be limited by theory, the low temperature sintering is believed to becaused by the halide ions forming a metal halide along the surface,which promotes metal ion migration resulting in the sintering of themetal cores of adjacent metal nanowires. The results in the examplestrongly suggest connection of the metal cores in the sinteredmaterials. Processing can be performed at low temperatures, such as atroom temperature.

The formation of the sintered network also results in a core shellstructure. Results suggest that the shell is a metal halide that islocated on top of the metal core. The details of the shell propertieswould be expected to depend on the specific processing conditions, butgenerally the shell has an average thickness from about 1 nm to about 10nm. A person of ordinary skill in the art will recognize that additionalranges of shell thickness within the explicit range above arecontemplated and are within the present disclosure. The metal halideshell may influence some properties of the fused metal network since theshell has intrinsically different optical properties from the metalcore. In particular, metal wires reflect visible light, while metalhalides generally have low reflectivity with respect to visible light.Furthermore, the metal halides have a lower reflectivity relative to thecore metal. The optical properties of the shell may be advantageous insome contexts. Also, the metal halides are generally soluble in solventsthat do not affect the metal core, so that it should be possible toselectively remove the metal halide shell without disturbing the fusedmetal network.

As noted above the amount of nanowires delivered onto the substrate caninvolve a balance of factors to achieve desired amounts of transparencyand electrical conductivity. While thickness of the nanowire network canin principle be evaluated using scanning electron microscopy, thenetwork can be relatively fragile, which can complicate the measurement.In general, the fused metal nanowire network would have an averagethickness of no more than about 5 microns. However, the fused nanowirenetworks are generally relatively open structures with significantsurface texture on a submicron scale, and only indirect methods cangenerally be used to estimate the thickness. The loading levels of thenanowires can provide a useful parameter of the network that can bereadily evaluated, and the loading value provides an alternativeparameter related to thickness. Thus, as used herein, loading levels ofnanowires onto the substrate is presented as microgram or milligrams ofnanowires for a square centimeter of substrate. In general, the nanowirenetworks can have a loading from about 0.1 microgram/cm² to about 5milligrams (mg)/cm², in further embodiments from about 1 microgram/cm²to about 2 mg/cm², and in other embodiments from about 5 microgram g/cm²(μg/cm²) to about 1 mg/cm². A person of ordinary skill in the art willrecognize that additional ranges of thickness and loading within theexplicit ranges above are contemplated and are within the presentdisclosure.

Electrical conductivity can be expressed as a sheet resistance, which isreported in units of ohms per square (Ω/□ or ohms/sq) to distinguish thevalues from bulk electrical resistance values according to parametersrelated to the measurement process. Sheet resistance of films isgenerally measured using a four point probe measurement or an equivalentprocess. In the Examples below, film sheet resistances were measuredusing a four point probe, or by making a square using a quick dryingsilver paste. The fused metal nanowire networks can have a sheetresistance of no more than about 200 ohms/sq, in further embodiments nomore than about 100 ohms/sq, and in other embodiments no more than about60 ohms/sq. A person of ordinary skill in the art will recognize thatadditional ranges of sheet resistance within the explicit ranges aboveare contemplated and are within the present disclosure. In general,sheet resistance can be reduced by increasing the loading of nanowires,but an increased loading may not be desirable from other perspectives asdescribed further below, and the loading is not as significant asachieving good fusing for improving the sheet resistance.

For applications as transparent conductive films, it is desirable forthe fused metal nanowire networks to maintain good optical transparency.In general, optical transparency is inversely related to the loading,although processing of the network can also significantly affect thetransparency. The optical transparency can be evaluated relative to thetransmitted light through the substrate. For example, the transparencyof the conductive film described herein can be measured by using aUV-Visible spectrophotometer and measuring the total transmissionthrough the conductive film and support substrate. Transmittance is theratio of the transmitted light intensity (I) to the incident lightintensity (I_(o)). The transmittance through the film (T_(film)) can beestimated by dividing the total transmittance (T) measured by thetransmittance through the support substrate (T_(sub)). (T=I/I_(o) andT/T_(sub)=(I/I_(o))/(I_(sub)/I_(o))=I/I_(sub)=T_(film)) While it isgenerally desirable to have good optical transparency across the visiblespectrum, for convenience, optical transmission is reported herein at550 nm wavelength of light. In some embodiments, the film formed by thefused network has a transmission at 550 nm of at least 80%, in furtherembodiments at least about 85% and in additional embodiments, at leastabout 90%. Transparency of the films on a transparent polymer substratecan be evaluated using the standard ASTM D1003 (“Standard Test Methodfor Haze and Luminous Transmittance of Transparent Plastics”),incorporated herein by reference. As noted above, the correlation ofgood optical transparency with low electrical resistance can beparticularly desirable. In some embodiments with a sheet resistance from20 ohm/sq to about 150 ohm/sq, the films can have an opticaltransmission at 550 nm of at least about 86%, in further embodiments atleast about 88% and in other embodiments from about 89% to about 92%. Inone embodiment, the film can have a sheet resistance of no more thanabout 75 ohm/sq and a transparency of at least about 85% at 550 nm. Inanother embodiment, the film can have a sheet resistance of no more thanabout 175 ohm/sq and a transparency of at least about 90% at 550 nm. Aperson or ordinary skill in the art will recognize that additionalranges of optical transmission within the explicit ranges above arecontemplated and are within the present disclosure.

The sintered metal networks can also have low haze along with hightransmission of visible light while having desirably low sheetresistance. Haze can be measured using a hazemeter based on ASTM D1003referenced above. In some embodiments, the sintered network film canhave a haze value of no more than about 0.5%, in further embodiments nomore than about 0.45% and in additional embodiments no more than about0.4%. A person of ordinary skill in the art will recognize thatadditional ranges of haze within the explicit ranges above arecontemplated and are within the present disclosure.

As described in the Examples below, the processing approaches describedherein result in the fusing of the metal nanowires. This fusing isbelieved to contribute to the enhanced electrical conductivity observedand to the improved transparency achievable at low levels of electricalresistance. The fusing is believed to take place at points of nearcontact of adjacent nanowires during processing. Thus, fusing caninvolve end-to-end fusing, side wall to side wall fusing and end to sidewall fusing. The degree of fusing may relate to the processingconditions. As described further below, short processing times arebelieved to contribute good fusing without degradation of the nanowirenetwork.

In general, suitable substrates can be selected as desired based on theparticular application. Substrate surfaces can comprise, for example,polymers, glass, inorganic semiconductor materials, inorganic dielectricmaterials, polymer glass laminates, composites thereof, or the like.Suitable polymers include, for example, polyethylene terephthalate(PET), polyacrylate, polyolefins, polyvinyl chloride, fluoropolymers,polyamides, polyimide, polysulfones, polysiloxanes,polyetheretherketones, polynorbornenes, polyester, polyvinyl alcohol,polyvinyl acetate, acrylonitrile-butadiene-styrene copolymer,polycarbonate, copolymers thereof, mixtures thereof and the like.Furthermore, the material can have a polymer overcoat placed on thefused metal nanowire network, and the overcoat polymers can comprise thepolymers listed for the substrates above. Moreover, other layers can beadded on top or in between the conductive film and substrate to reducereflective losses and improve the overall transmission of the stack.

Processing of Nanowire Networks

The improved electrical conductivity and optical transparency has beenfound to be obtained with short time treatment of as deposited metalnanowire films with compounds comprising halogen anions. Desirableincreases in electrical conductivity have been achieved with both vapordelivery of the fusing composition or with solution based delivery. Thefusing achieves low electrical surface resistance while maintaining highlevels of optical transmission.

The formation of the metal nanowire network comprises the formation of adispersion of the metal nanowires in a suitable liquid and applying thedispersion as a coating onto the selected substrate surface. Theconcentration of the dispersion can be selected to obtain a gooddispersion of the nanowires to provide for a desired degree ofuniformity of the resulting coating. In some embodiments, the coatingsolution can comprise from about 0.1 wt % to about 5.0 wt % metalnanowires, and in further embodiments from about 0.25 wt % to about 2.5wt % metal nanowires. A person of ordinary skill in the art willrecognize that additional ranges of metal nanowire concentrations withinthe explicit ranges above are contemplated and are within the presentdisclosure. Similarly, the liquid for forming the dispersion can beselected to achieve good dispersion of the nanowires. For example,aqueous solvents, alcohols, such as ethanol or isopropyl alcohol, ketonebased solvents, such as methyl ethyl ketone, organic coating solvents,such as toluene or hexane, or the like or mixtures thereof, aregenerally good dispersants for metal nanowires.

Any reasonable coating approach can be used, such as dip coating, spraycoating, knife edge coating, bar coating, Meyer-rod coating, slot-die,gravure, spin coating or the like. After forming the coating with thedispersion, the nanowire network can be dried to remove the liquid. Thedried film of metal nanowires can then be processed to achieve nanowirefusing.

A first approach to fusing can be performed with acid halide vapor, suchas vapor from HCl, HBr, HI or mixtures thereof. HF can also be used, butHF may be corrosive to some substrate materials and is more toxic.Specifically, the dried coating can be exposed to the vapor of the acidhalide for a brief period of time. The hydrogen halide compounds aregaseous and are soluble in water and other polar solvents such asalcohol. Generally, the vapor for fusing the metal nanowire film can begenerated from a gas reservoir or from vapor given off by solutions ofthe hydrogen halide compounds. Acidic vapors can quickly be passed overthe coating surfaces for example for about 10s to form the nanowirenetwork. In general, the coating containing the nanowires can be treatedwith acid vapor for no more than about 4 minutes, in further embodimentsfor from about 2 seconds to about 3.5 minutes and in other embodimentsfrom about 5 seconds to about 3 minutes. A person of ordinary skill inthe art will recognize that additional ranges of treatment times arecontemplated and are within the present disclosure.

In further embodiments, the initial metal nanowires can be fused with asolution comprising halide anions. In particular, the solutioncomprising dissolved acid halide, dissolved metal halide salts or acombination thereof. Suitable compositions for forming the halidesolutions include, for example, HCl, HBr, HF, LiCl, NaF, NaCl, NaBr,NaI, KCl, MgCl₂, CaCl₂, AlCl₃, NH₄Cl, NH₄F, AgF, or a combinationthereof. In particular NaCl, NaBr, and AgF provide particularlydesirable fusing properties. In general, the halide fusing solution canbe added to a previously formed coating comprising the metal nanowiresto fuse the metal nanowires. Additionally or alternatively, the halidecomposition can be combined with the metal nanowire dispersion that isthen deposited as a coating so that the metal nanowires and the fusingagent are simultaneously deposited. If the fusing agent is included withthe metal nanowires in the metal nanowire dispersion, a separate fusingsolution can also be delivered onto the metal nanowire coating to add anadditional quantity of fusing agent.

The solutions for separate application of the fusing agent generallycomprise halide ions at concentrations of at least about 0.01 mM, insome embodiments, from about 0.1 mM to about 10M, in further embodimentsfrom about 0.1 M to about 5 M. The metal nanowires can be contacted withthe halide solution using any reasonable approach such as dip coating,spraying, or the like. Alternatively or additionally, the halide salt oracid can be added directly to dispersant of nanowires in ranges from0.01 mM to about 1M, in further embodiments from about 0.05 mM to about50 mM and in additional embodiments from about 0.5 mM to about 10 mM, toform a nanowire and halide mixture. The mixture is then coated onto thesubstrate surface as described above to form a coating. The filmformation process then results in the direct formation of the film withthe fusing agent already present. Whether the solution comprising halideanions is delivered with the metal nanowire coating solution, with aseparate fusing solution or both, the nanowires in the coating formfused nanowire networks upon solvent removal and the saturation of thehalide ions. Formation of the nanowire network is complete when thesolvent is completely removed from the coating to form a dry film, andwhile not wanting to be limited by theory, the fusing process isbelieved to be related to the concentration of the halide anions duringthe drying process. A person of ordinary skill in the art will recognizethat additional ranges of concentration within the explicit ranges aboveare contemplated and are within the present disclosure.

The chemical reaction that results in the fusing of the metal nanowiresinto a network can be performed at room temperature, although it is notnecessary to be performed at room temperature. Thus, the structure canbe cooled or heated reasonably, generally without significantly changingthe resulting structure. For embodiments in which the sintering agent isdelivered in a solution, the solvent is evaporated as part of theprocess, and some heating can be desirable to speed the drying process,although the drying can be performed slowly by evaporation at roomtemperature or lower temperatures with or without reducing the pressureover the film to speed evaporation. The changing of the temperatureinvolves the expenditure of energy, so it is generally desirable to notexcessively heat or cool the materials even if the results of theprocessing are not greatly changed. The processing temperatures can bekept well below any temperatures at which any components, such as thesubstrate, melt or otherwise are adversely affected. In summary, theprocessing described herein generally can be performed at or near roomtemperature and other reasonable temperatures, and the processingtemperatures can generally be selected to be low relative to meltingtemperatures of the materials involved. A selected temperature can beinfluenced by practical issues such as processing cost, processingequipment and processing time.

After completing the fusing process, the fused metal nanowire networksare ready for any additional further processing to form the finalproduct. For example, the coating or film containing the metal nanowirenetworks may be rinsed to remove unreacted sintering agents, and/or maybe encapsulated with a protective coating. Due to the high transparencywith low electrical resistance, the fused nanowire networks are wellsuited for the formation of transparent conductive electrodes,transparent composites, which can be used for solar cells, displays,touch screens, solar windows, capacitive switches, and the like.

Patterning

The processing approaches described herein can be used for efficientpatterning of films to form patterns of electrically conductive regionsand less conductive regions with desirable optical transparency acrossthe film. In particular, since the sintering process is performedchemically, the controlled delivery of the sintering agent to selectedportions of a metal nanowire film can form a sintered metal network atthe portions of a film contacted with the sintering agent, while theremaining portions of the metal nanowire film remain un-sintered. Ofcourse, control of the sintering agent delivery does not have to beperfect for the patterning to be effective for appropriate applications.

The particular pattern of sintered conductive network along thesubstrate surface generally is guided by the desired product. Theproportion of the surface comprising the electrically conductivesintered network can generally be selected based on the selected design.In some embodiments, the sintered network comprises from about 1 percentto about 99 percent of the surface, in further embodiments from about 5percent to about 85 percent and in additional embodiment from about 10percent to about 70 percent of the substrate surface. A person ofordinary skill in the art will recognize that additional ranges ofsurface coverage within the explicit ranges above are contemplated andare within the present disclosure. The sintered network along thesurface can form a conductive pattern with a single pathway 21, as shownin FIG. 1H or with a plurality of electrically conductive pathways 23,25, and 27, as shown in FIG. 1I. As shown in FIG. 1I, the sintered areaforms three distinct electrically conductive regions 23, 25, and 27.Although a single connected conductive region and three independentlyconnected conductive regions have been illustrated in the figures, it isunderstood that patterns with two, four or more than 4 conductiveindependent conductive pathways or regions can be formed as desired.Similarly, the shapes of the particular conductive regions can beselected as desired.

The difference between the electrical conductivity of the sinterednetwork regions of the surface and the un-sintered nanowire regions canprovide desired functionality. In general, the variation in theelectrical conductivity between the sintered regions and the un-sinteredregions can be very large, as described in the examples, although lesslarge contrasts can still be effective. In general, the un-sinteredmetal nanowire regions have a sheet resistance that is at least about 5times the sheet resistance of the sintered metal network, in furtherembodiments at least about 100 times, in additional embodiments at leastabout 1000 times, and in other embodiments at least about 1,000,000 orgreater times the sheet resistance of the sintered metal network. It canbe difficult to measure extremely high resistances due to cut offs inthe measurement scale. A person of ordinary skill in the art willrecognize that additional ranges within the explicit ranges above arecontemplated and are within the present disclosure. The opticaltransparency to visible light can be approximately the same through thesintered metal network and through the un-fused metal nanowire film,although other optical properties of the sintered network regions of thefilm can be different from the un-sintered regions due to the core-shellstructure of the sintered networks.

The patterning of sintered and un-sintered regions of the metal nanowirefilm can be driven by the selective delivery of the sintering agent.Thus, the metal nanowire film can be first delivered to a surface. Ingeneral, the metal nanowire film can be delivered to be relativelyuniform across the surface or some appropriate portion thereof. Ofcourse, a fraction of the surface can remain uncoated at all withnanowire film, and references to patterning refer to the portions of thesurface with the nanowire film, i.e. sintered and un-sintered portionsof the film.

With the use of a vapor sintering agent, such as HCl vapor, a portion ofthe substrate selected to remain un-sintered, is masked or otherwiseblocked from contact with the vapors. Then, the unmasked portions of themetal nanowire film are contacted with the sintering vapor to form thesintered metal network. The mask or other cover can be removed followingthe completion of the contact with the sintering agent.

If a liquid solution comprising the sintering agent is applied to themetal nanowire film, the sintering solution can be delivered to theselected portions of the film to perform the sintering. While a wellsealed mask can be used to prevent contacting of the liquid sinteringagent with selected portions of the film, it can be desirable to printthe liquid sintering agent along the desired portion of the film usingink jet printing, screen printing or other appropriate printing process.The properties of the liquid sintering agent can be adjusted to beappropriate for the particular printing approach. A small volume ofliquid sintering agent can be delivered to provide the appropriatesintering. The liquid and/or the printing process can be controlled tolimit the spreading of the sintering liquid or to have spreadingcontrolled to provide sintering over a selected region.

The efficient patterning of the conductive transparent film can be veryeffective for certain display and or touch sensor applications. Inparticular, a touch sensor may desirably have patterns of electricallyconductive regions to provide for a corresponding pattern of touchsensors, and the transparency provides for the visualization of adisplay or the like under the pattern as shown in FIGS. 1F and 1G above.The use of patterned transparent conductive electrodes for the formationof patterned touch sensors is described, for example, in U.S. Pat. No.8,031,180 to Miyamoto et al., entitled “Touch Sensor, Display With TouchSensor, and Method for Generating Position Data,” and published U.S.patent application 2012/0073947 to Sakata et al., entitled “Narrow FrameTouch Input Sheet, Manufacturing Method of Same, and Conductive SheetUsed in Narrow Frame Touch Input Sheet,” both of which are incorporatedherein by reference.

EXAMPLES

Silver nanowires with different sizes purchased from either ACSMaterials or Seashell Technology, LLC (CA, USA) were used in thefollowing examples. The properties of the silver nanowires were anaverage diameter of 60 min and an average length of 10 microns or anaverage diameter of 115 nm and an average length of 30 microns.

Example 1 Fabrication of Transparent Conductive Material Using HCl VaporTreatment

This example demonstrates the ability to use a vapor based fusing agentto chemically drive the fusing of silver nanowires to dramaticallyimprove the electrical conductivity.

Commercially available silver nanowires (AgNWs) were dispersed inalcohols e.g. ethanol or isopropanol to form an AgNWs dispersion. TheAgNWs dispersions were typically in the 0.1-1.0% wt range. Thedispersion was then deposited on glass or polyethylene terephthalate(PET) surfaces as an AgNWs film using a spray coating or a hand-drawnrod approach. The AgNWs film was then exposed briefly to HCl vapour as afusing agent. Specifically, the AgNWs film was exposed to HCl vapourfrom a concentrated HCl solution at room temperature for about 10seconds. AgNWs from two different vendors were used. The sheetresistance and transparency of the AgNWs film before and after thetreatment with HCl vapour were measured and recorded. The data of AgNWsfrom the first vendor is listed in Table 2 and the date of AgNWs fromthe second vender is listed in Table 3 below.

TABLE 2 Sample Sheet Resistance Before HCl Sheet Resistance After HClNo. (ohm/sq) (ohm/sq) 1 10000000 660 2 83000 60 3 10000000 1909 410000000 451 5 800000 113.4 6 695000 30 7 10000000 62 8 399000 562 914,200 53.4 10 10000000 283 11 10000000 1260 12 10000000 364 13 100000006700 14 10000000 1,460 15 10000000 70.5 16 10000000 2280 17 10000000 15518 10000000 1654 19 10000000 926

TABLE 3 Sheet Resistance Sample Sheet Resistance Before HCl (ohm/sq)After HCl (ohm/sq) 1 13180 253 2 6200000 244 3 6030 115 4 32240 43.6 54300000 68.3 6 10000000 1060 7 10000000 47.5 8 3790 61.7 9 4690 42.4 10404 37.5

Because the large numerical range involved, the data were plotted inlogarithmic format in figures so the small numbers can also bevisualized graphically. The data from Table 2 was plotted in FIG. 2 anddata from Table 3 was plotted in FIG. 3. The films corresponding to theelectrical conductivity results in Tables 2 and 3 had moderate loadingswith corresponding reasonable transparency to visible light. As shown inFIG. 2, the conductivity of the AgNWs film improved over 4 to 5 ordersof magnitude after the HCl vapour treatment. Additionally, these AgNWsfilms showed transparencies at 550 nm greater than 75%, which decreasedless than 0.5% after HCl vapor treatment. Similarly, in FIG. 3, dramaticimprovement in conductivity was also observed. The properties of thenanowire networks after fusing were relatively independent of theproperties of the initial nanowires for these two sets of nanowires, butthe longer nanowires exhibited overall a reduced electrical resistanceprior to fusing.

Additional AgNWs films were formed that has transparencies at 550 nmgreater than 85%. These films were also treated with HCl vapor for about10 seconds, and the sheet resistances of the AgNWs films before andafter the HCl vapour treatment were measured. The results for one set ofsamples are presented in Table 4, and results for another set of samplesare plotted in FIG. 4. Samples 2, 3, and 4 in FIG. 4 in particular havesheet conductivity between 30 to 50 ohm/sq while maintaining thetransparency of the films above 85%. The results shown in Table 4clearly demonstrate the ability to obtain transmission with 550 nm lightgreater than 90% with sheet resistance values less than 50 ohm/sq.

TABLE 4 Resistance Prior to Resistance After Transmission at 550 nmSintering Sintering (Conductive Film Only) 801 45 89.1 >10⁶ 40 88.9 >10⁶33 88.1 >10⁶ 20 87.8 >10⁶ 46 90.6 >10⁶ 182 92.4 >10⁶ 129 91.6 >10⁶ 8589.2

Example 2 Observation of the Fusing of the Silver Nanowires

This example provides evidence of nanowire physical fusing as a resultof contact with chemical fusing agents.

The dramatic conductivity improvement observed in Example 1 can beattributed to the fusing of some of the silver nanowires with adjacentsilver nanowires. Scanning electron micrographs (SEM) of the silvernanowires before treatment were obtained and are shown in FIG. 5. Asshown in FIG. 5, some of the ends (indicated by the circles) of thesilver nanowires appear to touch each other, but the ends apparently donot appear to be fused together. Additionally, polyvinylpyrrolidone(PVP) coating (indicated by arrows in the figure) can be seen to bepresent around the rods. As a comparison, the silver nanowires shown inFIG. 5 were heated at 100° C. for 10 minutes. No appreciableconductivity change has been observed after the heating. SEM micrographsof the silver nanowires after the heat treatment were obtained and areshown in FIG. 6. Heating does not appear to have fused the ends as shownin FIG. 6, some of the ends (indicated by the circles) of the silvernanowires do not appear to be fused together. Scanning electronmicrographs were obtained for nanowire networks after the HCl vaportreatment and are shown in FIG. 7. SEM of the silver nanowires in FIG. 7after the HCl treatment showed the ends (indicated by the circles) ofthe silver nanowires have been fused together, and other locations ofcontact between adjacent nanowires are believed to similarly fuse toform fused silver nanowire networks.

Example 3 Fabrication of Transparent Conductive Material Using HalideSolution Treatment

This example demonstrated the reduction in electrical resistance throughthe treatment of the networks with solutions containing halide anions.

Specifically, 50 mM solutions of AgF or NaCl in ethanol were used totreat the AgNWs films. When the fusing agent solution is used, the AgNWsfilm was submerged or covered with the fusing agent solution for about10 to about 30 seconds, or dilute solutions of AgF or NaCl were spraycoated (from ethanol) onto the AgNW. The AgNWs were then allowed to dry.The sheet resistance of the AgNWs film before and after the treatmentwith the halide solutions were measured and the results are shown inFIG. 8. As shown in FIG. 8 dramatic conductivity improvement is alsoobserved of the AgNWs films treated with halide solutions, with AgFtreated samples showing even more pronounced improvement compared to theNaCl treated samples. In general, the transmission of light changedmarginally (<5%) and less than 1% if residual salt solution was removed.Residual salt was removed by spraying gently with water or ethanol.

Dramatic improvements in conductivity with negligible changes intransparency are important for transparent conductor applications. Theconductivity of transparent conductors is often improved by adding moreconducting materials, for example more AgNWs, but the transmission candramatically decrease. The methods and processes described hereinprovide a convenient and cost effective approach to dramatically improvethe conductivity of nanowire materials without sacrificing transparencyor adding additional nanowires.

Example 4 Low Haze Transparent Conductive Material Using HCl Vapor

This example demonstrates the ability to use HCl Vapor to bond silvernanowires to form core-shell structures that have dramatically improvedelectrical conductivity while having low haze and maintainingtransparency.

Commercially available silver nanowires (AgNWs) roughly 35 nm indiameter and 15-20 microns in length were dispersed in alcohols e.g.ethanol or isopropanol to form an AgNWs dispersion. The AgNWsdispersions have typically about 0.2 wt % concentration. The dispersionwas then cast using a draw down Meyer Rod (rod 10) onto polyethyleneterephthalate (PET) surfaces as AgNWs film samples 31 to 41 The AgNWsfilm samples were then exposed to HCl vapour for about 5 seconds tosinter the AgNWs together to form a core-shell formulation. Sheetconductivity of the AgNWs film samples before and after the sinteringprocess was measured using an R-Check hand-held 4 point probe or bymaking a perfect square and painting silver paste. The totaltransmission (TT) and haze of the AgNWs film samples were measured usinga BYK Gardner Haze Meter. The BYK instrument is designed to evaluateoptical properties based on ASTM D 1003 standard. The data of AgNWssamples 31-41 are listed in Table 5 below. Percentage of transmission (%T) and haze were obtained from the conductive film only, i.e. theoptical properties of the bare polymer substrate are subtracted away.The conductivities of the film samples before and after the sinteringwere plotted in FIG. 9 with the conductivity in logarithmic scale. Asshown in FIG. 9, dramatic improvement in conductivity was observed afterthe treatment with the vapor sintering agent.

TABLE 5 Pre-sintering Post Sintering Sample R_(s) Ω/□ % T R_(s) Ω/□ % THaze 31 >10⁴ 99.1 135 99.0 0.65 32 >10⁴ 99.1 128 99.1 0.61 33 >10⁴ 99.0178 98.9 0.10 34 >10⁴ 98.2 204 98.1 0.23 35   10³ 98.9 120 98.7 0.43 36  10³ 98.9 191 98.9 0.46 37   10³ 99.0 122 99.0 0.34 38 >10⁴ 99.4 10599.1 0.47 39 >10³ 98.1 150 98.1 0.92 40 >10⁴ 98.5 198 98.5 0.53 41   10³98.6 306 98.6 0.52

Example 5 Transparent Conductive Material Using Fluoride Salt SeparatelyAdded as Sintering Solution

This example demonstrates the ability to use a fluoride salt as asintering agent to bond silver nanowires to form core-shell structuresthat have dramatically improved electrical conductivity whilemaintaining high transparency.

Commercially available silver nanowires (AgNWs) roughly 40 nm indiameter and 15-20 microns in length were dispersed in alcohols e.g.ethanol or isopropanol to form an AgNWs dispersion. The AgNWsdispersions have typically about 0.2 wt % concentration. The dispersionwas then cast using a draw down Meyer Rod onto polyethyleneterephthalate (PET) surfaces as AgNWs film samples 42 and 43. Eachsample 42 and 43 were formed with triplicates. The AgNWs film samples 42and 43 were then submerged into LiF or NaF solutions respectively forabout 5 seconds to carry out the sintering process. The concentrationsof the LiF or NaF solutions were about 1.0 mM. The submerged films werethen dried with nitrogen to form the core-shell metal network. Theconductivity of the film samples 42 and 43 before and after thesintering was plotted in FIGS. 10A and 10B respectively with theconductivity in logarithmic scale. The films formed have hightransparency generally greater than about 85 percent.

Example 6 Transparent Conductive Material Using Mixture of AgNWs andFluoride Salt

This example demonstrates the mixture of fluoride salt and silvernanowires form core-shell structures that have dramatically improvedelectrical conductivity while having low haze and maintainingtransparency.

Commercially available silver nanowires (AgNWs) roughly 40 nm in averagediameter and 15-20 microns in average length were dispersed in alcoholse.g. ethanol or isopropanol to form an AgNWs stock dispersion. AgF andAlF₃ solutions with concentration of about 1.0 to 5.0 mM were created inalcohols such as isopropanol (IPA). The AgNWs stock dispersion was thenadded into the AgF and AlF₃ solutions to form mixture solutions 44 and45 respectively. The concentration of AgNWs in these mixture solutionswas about 0.2 wt %. The mixture solutions 44 and 45 were then cast usinga draw down Meyer Rod (rod 10) onto polyethylene terephthalate (PET)surfaces as AgNWs film samples 44 and 45. Each sample 44 and 45 wereformed with triplicates. The film samples 44 and 45 were then driedusing a heat gun for about 5 seconds to produce the core-shellformulation. Same procedure was carried out for each triplicate ofsamples 44 and 45. The conductivity of the film samples 44 and 45 beforeand after the sintering was plotted in FIGS. 11A and 11B respectivelywith the conductivity in logarithmic scale. The films had hightransparency, generally greater than about 85% and in some samplesgreater than about 89%.

Example 7 Analysis of Sintered Materials

Sintered materials from examples above have been analyzed and evaluatedto better understand the core-shell structure and sintering.

XPS (X-Ray Photoelectron Spectroscopy) was performed on sample 42 beforeand after the sintering and the results are shown in FIG. 12A and FIG.12B respectively. As indicated by the arrows and circles in FIG. 12B,shell material has formed after the sintering process. The intensity ofthe peaks corresponding to different elements in the samples werefurther analyzed and listed in Table 6 below.

TABLE 6 C N O F Si Ag Before sintering 68.1 1.7 25.3 — 4.2 0.7 Aftersintering 64.6 2.2 27.8 1.2 3.4 0.9

The sample was further analyzed by SEM and the results are shown inFIGS. 13A-13D with FIG. 13A showing the material before sintering andFIGS. 13B-13D showing the films after sintering. Difference inintensities (brightness) from SEM is indicative of materials withdifferent conductivities (a) and densities (p) with ρ_(core)˜1.7ρ_(shell), σ_(core)>>σ_(shell), based on the values of the bulkmaterials. Thus, the SEM photos provide confirmation of the presence ofa core-shell structure after performing the sintering.

The reflection off the core material only is about 0.96 in bulk whilethe reflection off the shell material is 0.12 in bulk. Bulk silverchloride material is known to have a refractive index of about 2. Thisis consistent with the observation made by Bi et al. in Chem. Commun.2009, 6551-6553 entitled “In situ oxidation synthesis of Ag/AgClcore-shell nanowires and their photocatalytic properties”, where theabsorption decreases as an AgNW is converted to an AgCl nanowire whichtranslates into high transmission with the conversion of Ag to Ag/AgClcore-shell type of structure. Bi et al. described a synthesis method forAgCl/Ag core-shell nanowire to study photocatalytic behavior of thesesynthesized materials. The process disclosed by Bi et al takes about 40min and no discussion about using the material as potential transparentconductor was made. Sun et al. in Materials Letters 61 (2007) 1645-1648entitled “AgCl nanoparticle nanowires fabricated by template method”described methods for making AgCl nanowires and AgClnanoparticle-nanowires. Sun et al. however, does not discuss theformation of a core-shell type structure or using the material aspotential transparent conductor. In fact, currently, there is noexisting publication on using AgF/Ag or AgCl/Ag core-shell materials fortransparent conductor applications. The core-shell material describedherein is expected to have better optics, reflection and haze introducedby core-shell structure.

Surface Plasmon Resonance of samples before and after HCl vaporsintering was measured and the results plotted in FIG. 14 as A and Brespectively for before and after samples. Reduction in Absorption(increase in transmission) has been observed for the after treatmentsample B due to the reduction of Ag surface Plasmon resonance in sampleB, which is consistent with the formation of the silver fluoride shell.

Example 8 AgNW Films with Patterned Areas Treated or not Treated withHCl Vapor

Half of AgNW film was treated with HCl and the properties of theuntreated half and the treated half were measured and compared.

Commercially available silver nanowires (AgNWs) roughly 35 nm indiameter and 15-20 microns in length were dispersed in alcohols e.g.ethanol or isopropanol to form an AgNWs dispersion. The AgNWsdispersions have typically about 0.2 wt % concentration. The dispersionwas then cast using a draw down Meyer Rod (rod 10) or blade coated(using ≈25 micron wet thickness) onto polyethylene terephthalate (PET)surfaces as AgNWs film samples 42 to 46 that are about 2×2 inches insize. Half of the sample film area “a”, about 1×2 inches was selectivelyexposed to HCl vapor for about 5 seconds while the other half of thesample film area “b” was protected from the HCl exposure.

The percentages of transmission (% T) at 550 nm and sheet resistance ofthe two areas were measured and the data are listed in Table 7 below.Sheet conductivity of the AgNWs film sample areas “a” and “b” with orwithout HCl exposure was measured using an R-Check hand-held 4 pointprobe or by making a perfect square and painting silver paste. The % Twas obtained from the AgNW film only, i.e. the optical properties of thebare polymer substrate are subtracted away. The % TT (TotalTransmission) of film is expected to be 1.5-2.0% higher than the % Tvalue. Aside from the very close values in % T, visibly it is verydifficult to distinguish between the sintered and non-sintered areas onthe same film. The conductivity of the areas “a” and “b” of the filmsamples were plotted in FIG. 15 with the conductivity in logarithmicscale. As shown in FIG. 15, the difference in resistance is >10⁴ ofareas “a” and “b” treated or not treated with HCl vapor sintering agent.

TABLE 7 Sample/Area Sintered? Ω/□ % T 42a NO >100k 88.2 42b YES   18788.1 43a NO >100k 88.2 43b YES   155 88.0 44a NO >100k 87.0 44b YES  112 87.2 45a NO >100k 87.9 45b YES    69 88.3 46a NO >100k 87.8 46bYES   152 88.4

The embodiments above are intended to be illustrative and not limiting.Additional embodiments are within the claims. In addition, although thepresent invention has been described with reference to particularembodiments, those skilled in the art will recognize that changes can bemade in form and detail without departing from the spirit and scope ofthe invention. Any incorporation by reference of documents above islimited such that no subject matter is incorporated that is contrary tothe explicit disclosure herein.

What is claimed is:
 1. A precursor ink comprising a solvent, at leastabout 0.01 weight percent metal nanowires and from about 0.05 mM toabout 50 mM halide anions.
 2. The precursor ink of claim 1 having fromabout 0.025 to about 2 weight percent metal nanowires.
 3. The precursorink of claim 1 having from about 0.25 mM to about 10 mM halide anions.4. The precursor ink of claim 1 wherein the metal nanowires are silvernanowires.
 5. The precursor ink of claim 4 wherein the silver nanowireshave an average diameter of no more than about 75 nm and a length of atleast about 5 microns.
 6. The precursor ink of claim 1 wherein thesolvent comprises alcohol.
 7. The precursor ink of claim 1 wherein thesolvent comprises water.
 8. A method for forming a transparentconductive film, the method comprising: depositing a precursor ink ofclaim 1; and drying the deposited ink to form the film.
 9. A materialcomprising a transparent conductive coating and a substrate on which thecoating is supported, the coating comprising a sintered metal nanowirenetwork comprising sintered metal nanowires, wherein the coating has atransparency to visible light of at least about 90%, a sheet resistanceof no more than about 500 Ohms/square, and a haze of no more than 0.5.10. The material of claim 9 wherein the coating has a sheet resistanceof no more than about 200 Ohms/square.
 11. The material of claim 9wherein the substrate comprises a polymer film.
 12. The material ofclaim 9 wherein the sintered metal nanowires comprise sintered silvernanowire.
 13. The material of claim 12 wherein the sintered metalnetwork comprises a metal halide shell layer.
 14. The material of claim9 wherein the coating has a transparency to visible light of at leastabout 95%.
 15. The material of claim 9 wherein the coating has a surfaceloading level of the sintered metal network from about 0.1micrograms/cm² to about 5 mg/cm².
 16. The material of claim 9 whereinthe coating has a haze of no more than about 0.4.
 17. A materialcomprising a transparent conductive coating and a substrate on which thecoating is supported, the coating comprising a sintered metal nanowirenetwork with a metal halide shell layer over a metallic core, whereinthe metal halide shell layer has an average thickness of at least about1 nanometer.
 18. The material of claim 17 wherein the sintered metalnanowire network comprises sintered silver nanowires.
 19. The materialof claim 17 wherein the coating has a transparency of at least about 90%of visible light.
 20. The material of claim 19 wherein the coating has asheet resistance of no more than about 200 Ohms/square.
 21. The materialof claim 18 wherein the silver nanowires incorporated into the sinterednetwork have an average diameter of no more than about 75 nm and anaverage length of at least about 5 microns and wherein the material hasa surface loading level of the sintered silver nanowires in the networkfrom about 0.1 micrograms/cm² to about 5 mg/cm².
 22. A patternedstructure comprising a substrate and a coating, wherein the coating ispatterned with un-sintered metal nanowires in a selected portion of thecoating and a sintered metal network over another portion of thecoating.
 23. The patterned structure of claim 22 wherein the coatingportion with the sintered metal nanowire network has a sheet resistanceof no more than about 500 Ohms/square and the portion with un-sinteredmetal nanowires has a sheet resistance at least about 5 times the sheetresistance of the sintered metal network.
 24. The patterned structure ofclaim 22 wherein the coating has a transparency to visible light of atleast about 85%.
 25. The patterned structure of claim 24 wherein thecoating has approximately the same transparency across the entirecoating.
 26. The patterned structure of claim 22 wherein the un-sinteredmetal nanowires have a sheet resistance at least about 1000 times thesheet resistance of the sintered metal network.
 27. The structure ofclaim 22 wherein un-sintered metal nanowires of the coating comprisessilver nanowires.
 28. A method for forming a patterned structure thatcomprises a substrate and a patterned coating, the method comprising:selectively contacting a selected portion of a metal nanowire coatingwith a sintering agent to form a patterned coating with the selectedportion having a sheet resistance at least about 5 times less than thesheet resistance of the unselected portions of the metal nanowirecoating.
 29. The method of claim 28 wherein the selective contactingcomprises directing a sintering vapor to the selected portion of thecoating to form the patterned structure.
 30. The method of claim 29further comprising blocking the vapor with a mask over unselectedportions.
 31. The method of claim 28 wherein the selective contactingcomprises directing a solution comprising a sintering agent at theselected portion of the coating to form the patterned structure.
 32. Atouch sensor comprising a first electrode structure and a secondelectrode structure spaced apart in a natural configuration from thefirst electrode structure, the first electrode structure comprising afirst transparent conductive electrode on a first substrate wherein thefirst transparent conductive electrode comprises a first sinterednanostructured metal network.
 33. The touch sensor of claim 32 whereinthe second electrode structure comprises a second transparent conductiveelectrode on a second substrate wherein the second transparentconductive electrode comprises a second sintered nanostructured metalnetwork.
 34. The touch sensor of claim 32 wherein the first electrodestructure and the second electrode structure are spaced apart by adielectric layer and further comprising a circuit connected to theconductive electrode structures that measures changes in capacitance.35. The touch sensor of claim 32 further comprising display componentsassociated with the substrate.
 36. The touch sensor of claim 32 whereinthe substrates are transparent sheets.
 37. The touch sensor of claim 32further comprising a circuit connected to the electrode structures thatmeasures changes in electrical resistance.